1. Field of the Invention
The present invention relates generally to structural members, and in particular to plastic structural members for utility pole components, railroad ties and other applications.
2. Description of the Prior Art
Structural members have been made from various materials in order to withstand the environments in which they are installed. For example, utility poles are in widespread use for suspending utility lines, including electrical power, telephone/data, etc., at safe distances above the ground. Utility pole structural members have traditionally been manufactured predominately of wood, which has the advantages of being relatively ubiquitous, inexpensive, nonconductive, and generally at least adequate as a structural material with desired strength characteristics.
Other structural compositions for utility pole structural members include reinforced concrete, prestressed concrete, steel, aluminum, fiberglass and rigid plastic. For example, the Papin U.S. Pat. No. 5,775,035 discloses a power pole composed of rigid plastic structural members. The Sakai et al. U.S. Pat. No. 5,725,940 discloses a structure and method for making composite molded articles. The structures disclosed therein are generally molded, pulltruded and cast. They can be constructed with hollow, foam-filled and fiberglass honeycomb cores. Such structures and their manufacturing techniques tend to be relatively expensive or suffer from other drawbacks and disadvantages.
Disadvantages of wood include its susceptibility to damage from insects, birds, termites, etc. Wood is also subject to attack by biological organisms, particularly in humid environments. Still further, wood tends to deteriorate when exposed to the elements, such as ultraviolet radiation, precipitation, humidity, temperature cycles, etc. These and other factors have the cumulative effect of reducing the useful lives of wooden structural members which are exposed to the elements.
Disadvantages are also associated with other structural materials. For example, metals are subject to rust and corrosion. Moreover, their electrical conductivity generally makes them unsuitable for electrical utility pole construction requiring insulative properties. Materials which are hollow or porous are often unsuitable for exterior construction because they tend to admit water which can damage the structural members. Moreover, water permeation tends to increase electrical conductivity, making such materials unsuitable for electrical utility structures.
Other disadvantages associated with prior art structural materials include susceptibility to deformation and breaking when subjected to significant loads, both static and dynamic. Special tools and assembly techniques associated with some structural members can further increase the cost of construction and maintenance. Still further, a common practice involves coating exposed wooden structural members with a preservative, such as creosote. However, environmental laws and regulations significantly limit the permitted uses of wood preservatives, particularly those that contain toxins.
Plastic is often used as a replacement material for wood. For example, recycled plastic/composite railroad crossties have been substituted for wooden railroad crossties.
Although plastic materials tend to repel or resist water and are nonconductive, their disadvantages include vulnerability to ultraviolet radiation, higher densities as compared to wood, and cost. The present invention addresses some or all of the disadvantages and limitations associated with prior art structural members, including wooden and plastic utility pole crossarms and crossarm assemblies.
Heretofore there have not been available structural members or methods of manufacturing same with the advantages and features of the present invention.
In the practice of the present invention, a structural member, such as a utility pole crossarm, is manufactured from a plastic material and has a relatively dense outer surface and a less dense core. A crossarm assembly includes a plastic crossarm and a pair of diagonal braces for supporting the crossarm on the utility pole. Other utility pole components adapted for plastic construction according to the present invention include cross braces, insulators and the poles themselves. The structural members can be homogenous throughout with respect to their material composition. Other structural member applications include railroad ties.
A method of manufacturing the structural members includes extruding a continuous band comprising a polypropylene base material, a fiber reinforced plastic fill material, and a blowing or foaming agent. Additional materials can include chopped glass fibers, UV inhibitors, light stable pigments, antioxidant additives, processing aids, recycled materials, and endothermic gas producing agents. The materials are combined and extruded to form a continuous band, which is shaped and cooled in several stages and cut to predetermined lengths to form the structural members.
The principal objects and advantages of the present invention include: providing plastic structural members; providing such structural members which are adaptable to various applications; providing such structural members which are usable in constructing a wide variety of external structures; providing such structural members which are resistant to static and dynamic loads; providing such structural members which are resistant to environmental damage; providing such structural members which can be cut, drilled and assembled in the field using common tools and procedures; providing such structural members which meet all applicable regulatory requirements; providing such structural members which are adaptable for use as railroad ties; providing a plastic crossarm for utility poles; providing such a crossarm which is resistant to the elements; providing such a crossarm which is resistant to pest damage; providing such a crossarm which meets or exceeds the strength specifications for wooden crossarms; providing such a crossarm which weighs approximately the same amount as a comparable wood crossarm; providing such a crossarm which can be cut, drilled, etc. with tools used for working on wooden crossarms; providing such a crossarm which utilizes recycled plastic; providing a crossarm assembly with a plastic crossarm and plastic braces; providing such a crossarm assembly which is capable of passing lightning impulse tests associated with utility pole applications; providing such a crossarm assembly which meets or exceeds electrical insulation levels required for utility pole applications; providing such a crossarm which is economical to manufacture, efficient in operation, capable of a long operating life and particularly well adapted for the proposed usage thereof; and providing a method of manufacturing plastic structural members.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof